Looking at the dilemmas covered in previous sections, the following conclusions
emerge:

a carefully crafted portfolio of mitigation, adaptation, and learning activities
appears to be appropriate over the next few decades to hedge against the risk
of intolerable magnitudes and/or rates of climate change (impact side) and
against the need to undertake painfully drastic emission reductions if the
resolution of uncertainties reveals that climate change and its impacts might
imply high risks;

the nature of the climate change problem requires that mitigation action
at any level needs to start in the near term, as well as the development of
appropriate adaptation strategies;

emission reduction is an important form of mitigation, but the mitigation
portfolio includes a broad range of other activities, including investments
to develop low-cost non-carbon energy, and to improve energy efficiency and
carbon management technologies to make future CO2 mitigation inexpensive;

timing and composition of mitigation measures (investment in technological
development or immediate emission reductions) is highly controversial because
of the technological features of energy systems, and the range of uncertainties
involved with, for example, their impacts of climate change;

international flexibility instruments help reduce the costs of emission
reductions, but they raise a series of implementation and verification issues
that need to be balanced against the cost savings;

while there is a broad consensus on Pareto optimality as an efficiency principle,
there is no agreement on the best equity principle for burden sharing. Efficiency
and equity are important concerns in negotiating emissions limitation schemes,
and they are not mutually exclusive. Therefore, equity will play an important
role in determining the distribution of emissions allowances and/or within
compensation schemes that follow emissions trading resulting in a disproportionately
high level of burden to certain countries. Finally, it is more important to
rely on politically feasible burden-sharing rules than to select one specific
equity principle.

Finally, a series of potential large-scale geophysical transformations that
might exert a major influence on the desired level of stabilization have been
identified and examined more closely in recent years. These imply thresholds
that humanity might decide not to cross because the potential impacts or even
the associated risks are considered to be unacceptably high. Little is know
about these thresholds today. Most recent results and the implications of the
possibility of such thresholds are summarized in Chapter 19 of WGII (IPCC, 2001b).
Nevertheless, currently estimated danger zones are in the domain
of high stabilization levels for most threshold events.

Considering the special combination of features of the climate problem listed
at the beginning of this chapter, it is obvious that no once forever
solution exists. Making long-term commitments in any area where retraction is
possible is problematic. Making decisions that entail long-term and possibly
irreversible consequences due to long delays, inertia and similar system properties
is even more difficult, especially under severe uncertainties. Therefore, as
emphasized in this chapter, the most promising approach to climate policy is
sequential decision-making. This process involves a regular reassessment of
the long-term climate risks (net damages from a given magnitude of climate change)
and their management objectives (climate or GHG concentration stabilization)
in the light of newly available information. Short-term strategies are then
crafted so that both GHG emissions and the underlying socioeconomic processes
(resource use, technologies) evolve in a direction which makes future course
corrections in any direction the least expensive. The current structure of the
international climate regime is formulated in this vein: the UNFCCC provides
some, albeit vague, guidelines for long term stabilization objectives while
short-term goals are settled in and implemented under protocols for each budget
period.

The analytical tools to support the above decision-making processes need to
handle this double feature. They should provide policymakers with guidance to
set long-term targets and to formulate short-term policies and measures. Some
models take a long-term view to explore deep future impacts of climate change,
but this must not be interpreted as suggesting optimal strategies for the next
50-100-200 years. Other models explore what are the most promising near-term
policies and how to implement them. Similarly, many studies and models reviewed
in this chapter consider the world as a whole or broken down into a few regions,
at best. Others take a more detailed look at subnational and regional aspects.
They shed light on the smaller scale implications of climate change and its
management strategies, often in the context of other social concerns characterizing
the country or region. Our assessment has found a healthy diversity of DAFs
along both the long-term-short term and the global-local axes. Nevertheless,
the analytical capacity and thus quotable results are still badly missing in
most developing countries. This is probably the most severe problem to be solved
by the time the world community will prepare its next climate change assessment
report.